1. Technical Field
The present invention relates to a liquid ejecting head configured to eject liquid by driving piezoelectric elements and a liquid ejecting apparatus including the liquid ejecting head. In particular, the present invention relates to a liquid ejecting head and a liquid ejecting apparatus which are capable of reducing damage to piezoelectric elements.
2. Related Art
A liquid ejecting apparatus is an apparatus that includes a liquid ejecting head and that is configured to eject various kinds of liquid from the ejecting head. Examples of the liquid ejecting apparatus include image recording apparatuses such as an ink jet printer and an ink jet plotter. Recently, the liquid ejecting apparatus has been also used in various kinds of manufacturing equipment, making use of its advantage of being able to shoot a minute amount of liquid accurately to a target position. For example, the liquid ejecting apparatus has been used in a display producing apparatus for producing color filters for liquid crystal displays and the like, an electrode forming apparatus for forming electrodes for organic EL (electro luminescence) displays and FEDs (field emission displays) and the like, and a chip producing apparatus for producing biochips. A recording head for an image recording apparatus ejects liquid ink, whereas a color material ejecting head for a display producing apparatus ejects solutions of R (red), G (green), and B (blue) color materials. Furthermore, an electrode material ejecting head for an electrode forming apparatus ejects a liquid electrode material, whereas a living organic substance ejecting head for a chip producing apparatus ejects a solution of a living organic substance.
The liquid ejecting head is configured such that: liquid is introduced into pressure chambers; the liquid is subjected to a pressure change in the pressure chambers; and the liquid is ejected through nozzles in communication with the pressure chambers. The pressure chambers are formed in a crystalline substrate, such as that made of silicon, by anisotropic etching with dimensional accuracy. Piezoelectric elements are used to cause a pressure change in the liquid in the pressure chambers. A piezoelectric element may have a variety of configurations, and is constituted by, for example: a lower electrode layer which is positioned closer to a pressure chamber; a piezoelectric layer made from a piezoelectric material such as lead zirconate titanate (PZT); and an upper electrode layer, which are formed on top of each other by a film forming technique. For example, in a liquid ejecting head disclosed in FIG. 2 of JP-A-2011-126257, upper and lower electrodes are such that: a lower electrode layer is divided into separate electrodes corresponding to respective pressure chambers; and on the other hand, an upper electrode layer is a common electrode that is a continuous electrode extending across the pressure chambers. Since this configuration is employed, most of the piezoelectric layer is covered by the upper electrode layer. Therefore, the upper electrode layer also functions as a protection film, and the piezoelectric layer has increased moisture resistance. Furthermore, when seen in a direction intersecting the direction along which the pressure chambers are arranged, (when seen in the direction of the length of a pressure chamber), the layers constituting each piezoelectric element extend to outside the region of an opening of the pressure chamber. The reason therefor is as follows. Since the piezoelectric layer is provided over a wide range (entire surface) of a vibration plate with the lower electrode layer therebetween, the upper electrode layer, which extends to outside the region of the opening of each pressure chamber, can cover a wide area of the piezoelectric layer. In such a configuration, each portion in which the piezoelectric layer is sandwiched between the upper and lower electrodes serves as an active part that undergoes a deformation in response to application of voltage to the electrode layers. It should be noted that, in a configuration in which an upper electrode layer is divided into separate electrodes and a lower electrode layer is a common electrode, a moisture-proof protection film is separately provided to protect the piezoelectric layer from moisture. Therefore, the thickness of each piezoelectric element as a whole increases, and thus the piezoelectric element cannot undergo a displacement as efficiently as the earlier-mentioned configuration.
Meanwhile, in the liquid ejecting head having the earlier-mentioned configuration, the active part extends to outside of the region of the opening of each pressure chamber. Therefore, when a drive voltage is applied to the upper and lower electrode layers, an electric field occurs between the upper and lower electrodes also in a portion of the active part which is outside the region of the top opening of the pressure chamber, and this portion also tries to move. However, under this portion of the active part which is outside the region of the top opening of the pressure chamber, there exists a structure (that is, there exists a closed part of the substrate having pressure chambers, in which there are no pressure chamber openings). Therefore, this portion of the active part is fixed and cannot actually move. This causes a problem in that this portion of the active part experiences a large stress and may become broken or burnt. In addition, as compared to a configuration in which an upper electrode layer is divided into separate electrodes and a lower electrode layer is a common electrode, the piezoelectric elements are allowed a larger displacement. Therefore, stress may concentrate on a boundary between (i) a portion of the active part which corresponds to the region of the top opening of the pressure chamber, that is, a portion that can actually move, and (ii) a portion of the active part which is outside the region of the top opening of the pressure chamber, that is, a portion that cannot actually move. This may cause the piezoelectric elements to crack or even break.